1. Field of the Invention
This invention pertains to video processing and more specifically to a method and an apparatus for recording and playback of a field-length-modulated (FLM) video signal.
2. Description of the Prior Art
As known, FLM video signals cannot be satisfactorily recorded by consumer grade video cassette recorders (VCR's), but such signals can be displayed on a conventional television monitor or set. Field-length-modulation thus serves as one anti-copying measure for cable or broadcast television. FLM encoding essentially consists of varying the field length, e.g., the number of scan lines between vertical sync pulses, of video signals so that a VCR's head drum servomechanism cannot follow the fluctuations, whereas the electronic vertical deflection system of a television display monitor can follow. Variation of field length, typically by up to five scan lines, is performed so that the long-term average number of video lines per frame remains constant.
When an attempt is made to record and playback a FLM signal on a consumer grade VCR, the displayed video exhibits annoying instabilities thereby destroying the entertainment value of the recording and thus deterring pirating, i.e., the making of unauthorized copies. U.S. Pat. Nos. 4,488,176 to Bond, et al. and U.S. Pat. No. 4,673,981 to Lippman, et al. show exemplary FLM encoding systems for video signals.
One approach to defeat FLM encoding would be to remove the field-length-modulation by adding a correct number of scan lines to shortened fields and deleting a number of lines from lengthened fields. Though conceptually straight-forward, a system operating on this principle is relatively expensive requiring complex digital video processing circuitry. High cost and complexity provide a significant deterrent to attempts to defeat FLM video recordings.